In higher order eukaryotic organisms, DNA is methylated only at cytosines located 5′ to guanosine in the CpG dinucleotide. This modification has important regulatory effects on gene expression predominantly when it involves CpG rich areas (CpG islands) located in the promoter region of a gene sequence. Extensive methylation of CpG islands has been associated with transcriptional inactivation of selected imprinted genes and genes on the inactive X chromosome of females. Aberrant methylation of normally unmethylated CpG islands has been described as a frequent event in immortalized and transformed cells and has been frequently associated with transcriptional inactivation of tumor suppressor genes in human cancers.
DNA methylases transfer methyl groups from a universal methyl donor, such as S-adenosyl methionine, to specific sites on the DNA. One biological function of DNA methylation in bacteria is protection of the DNA from digestion by cognate restriction enzymes. Mammalian cells possess methylases that methylate cytosine residues on DNA that are 5′ neighbors of guanine (CpG). This methylation may play a role in gene inactivation, cell differentiation, tumorigenesis, X-chromosome inactivation, and genomic imprinting. CpG islands remain unmethylated in normal cells, except during X-chromosome inactivation and parental specific imprinting where methylation of 5′ regulatory regions can lead to transcriptional repression. DNA methylation is also a mechanism for changing the base sequence of DNA without altering its coding function. DNA methylation is a heritable, reversible and epigenetic change. Yet, DNA methylation has the potential to alter gene expression, which has profound developmental and genetic consequences.
The methylation reaction involves flipping a target cytosine out of an intact double helix to allow the transfer of a methyl group from S-adenosylmethionine in a cleft of the enzyme DNA (cystosine-5)-methyltransferase (Klimasauskas et al., Cell 76:357-369, 1994) to form 5-methylcytosine (5-mCyt). This enzymatic conversion is the only epigenetic modification of DNA known to exist in vertebrates and is essential for normal embryonic development (Bird, Cell 70:5-8, 1992; Laird and Jaenisch, Human Mol. Genet. 3:1487-1495, 1994; and Li et al., Cell 69:915-926, 1992). The presence of 5-mCyt at CpG dinucleotides has resulted in a 5-fold depletion of this sequence in the genome during vertebrate evolution, presumably due to spontaneous deamination of 5-mCyt to T (Schoreret et al., Proc. Natl. Acad. Sci. USA 89:957-961, 1992). Those areas of the genome that do not show such suppression are referred to as “CpG islands” (Bird, Nature 321:209-213, 1986; and Gardiner-Garden et al., J. Mol. Biol. 196:261-282, 1987). These CpG island regions comprise about 1% of vertebrate genomes and also account for about 15% of the total number of CpG dinucleotides (Bird, Nature 321:209-213, 1986). CpG islands are typically between 0.2 to about 1 kb in length and are located upstream of many housekeeping and tissue-specific genes, but may also extend into gene coding regions. Therefore, it is the methylation of cytosine residues within CpG islands in somatic tissues, which is believed to affect gene function by altering transcription (Cedar, Cell 53:3-4, 1988).
Methylation of cytosine residues contained within CpG islands of certain genes has been inversely correlated with gene activity. This could lead to decreased gene expression by a variety of mechanisms including, for example, disruption of local chromatin structure, inhibition of transcription factor-DNA binding, or by recruitment of proteins which interact specifically with methylated sequences indirectly preventing transcription factor binding. In other words, there are several theories as to how methylation affects mRNA transcription and gene expression, but the exact mechanism of action is not well understood. Some studies have demonstrated an inverse correlation between methylation of CpG islands and gene expression, however, most CpG islands on autosomal genes remain unmethylated in the germline and methylation of these islands is usually independent of gene expression. Tissue-specific genes are usually unmethylated in the receptive target organs but are methylated in the germline and in non-expressing adult tissues. CpG islands of constitutively-expressed housekeeping genes are normally unmethylated in the germline and in somatic tissues.
Abnormal methylation of CpG islands associated with tumor suppressor genes may also cause decreased gene expression. Increased methylation of such regions may lead to progressive reduction of normal gene expression resulting in the selection of a population of cells having a selective growth advantage (i.e., a malignancy).
It is considered that an altered DNA methylation pattern, particularly methylation of cytosine residues, causes genome instability and is mutagenic. This, presumably, has led to an 80% suppression of a CpG methyl acceptor site in eukaryotic organisms, which methylate their genomes. Cytosine methylation further contributes to generation of polymorphism and germ-line mutations and to transition mutations that inactivate tumor-suppressor genes (Jones, Cancer Res. 56:2463-2467, 1996). Methylation is also required for embryonic development of mammals (Li et al., Cell 69:915-926, 1992). It appears that the methylation of CpG-rich promoter regions may be blocking transcriptional activity. Ushijima et al. (Proc. Natl. Acad. Sci. USA 94:2284-2289, 1997) characterized and cloned DNA fragments that show methylation changes during murine hepatocarcinogenesis. Data from a group of studies of altered methylation sites in cancer cells show that it is not simply the overall levels of DNA methylation that are altered in cancer, but changes in the distribution of methyl groups.
These studies suggest that methylation at CpG-rich sequences, known as CpG islands, provide an alternative pathway for the inactivation of tumor suppressors. Methylation of CpG oligonucleotides in the promoters of tumor suppressor genes can lead to their inactivation. Other studies provide data that alterations in the normal methylation process are associated with genomic instability (Lengauer et al. Proc. Natl. Acad. Sci. USA 94:2545-2550, 1997). Such abnormal epigenetic changes may be found in many types of cancer and can serve as potential markers for oncogenic transformation, provided that there is a reliable means for rapidly determining such epigenetic changes. Therefore, there is a need in the art for a reliable and rapid (high-throughput) method for determining methylation as the preferred epigenetic alteration.
Methods to Determine DNA Methylation
There are a variety of genome scanning methods that have been used to identify altered methylation sites in cancer cells. For example, one method involves restriction landmark genomic scanning (Kawai et al., Mol. Cell. Biol. 14:7421-7427, 1994), and another example involves methylation-sensitive arbitrarily primed PCR (Gonzalgo et al., Cancer Res. 57:594-599, 1997). Changes in methylation patterns at specific CpG sites have been monitored by digestion of genomic DNA with methylation-sensitive restriction enzymes followed by Southern analysis of the regions of interest (digestion-Southern method). The digestion-Southern method is a straightforward method but it has inherent disadvantages in that it requires a large amount of high molecular weight DNA (at least or greater than 5 μg) and has a limited scope for analysis of CpG sites (as determined by the presence of recognition sites for methylation-sensitive restriction enzymes). Another method for analyzing changes in methylation patterns involves a PCR-based process that involves digestion of genomic DNA with methylation-sensitive restriction enzymes prior to PCR amplification (Singer-Sam et al., Nucl. Acids Res. 18:687, 1990). However, this method has not been shown effective because of a high degree of false positive signals (methylation present) due to inefficient enzyme digestion or overamplification in a subsequent PCR reaction.
Genomic sequencing has been simplified for analysis of DNA methylation patterns and 5-methylcytosine distribution by using bisulfite treatment (Frommer et al., Proc. Natl. Acad. Sci. USA 89:1827-1831, 1992). Bisulfite treatment of DNA distinguishes methylated from unmethylated cytosines, but original bisulfite genomic sequencing requires large-scale sequencing of multiple plasmid clones to determine overall methylation patterns, which prevents this technique from being commercially useful for determining methylation patterns in any type of a routine diagnostic assay.
In addition, other techniques have been reported which utilize bisulfite treatment of DNA as a starting point for methylation analysis. These include methylation-specific PCR (MSP) (Herman et al. Proc. Natl. Acad. Sci. USA 93:9821-9826, 1992); and restriction enzyme digestion of PCR products amplified from bisulfite-converted DNA (Sadri and Hornsby, Nucl. Acids Res. 24:5058-5059, 1996; and Xiong and Laird, Nucl. Acids Res. 25:2532-2534, 1997).
PCR techniques have been developed for detection of gene mutations (Kuppuswamy et al., Proc. Natl. Acad. Sci. USA 88:1143-1147, 1991) and quantitation of allelic-specific expression (Szabo and Mann, Genes Dev. 9:3097-3108, 1995; and Singer-Sam et al., PCR Methods Appl. 1: 160-163, 1992). Such techniques use internal primers, which anneal to a PCR-generated template and terminate immediately 5′ of the single nucleotide to be assayed. However an allelic-specific expression technique has not been tried within the context of assaying for DNA methylation patterns.
Most molecular biological techniques used to analyze specific loci, such as CpG islands in complex genomic DNA, involve some form of sequence-specific amplification, whether it is biological amplification by cloning in E. coli, direct amplification by PCR or signal amplification by hybridization with a probe that can be visualized. Since DNA methylation is added post-replicatively by a dedicated maintenance DNA methyltransferase that is not present in either E. coli or in the PCR reaction, such methylation information is lost during molecular cloning or PCR amplification. Moreover, molecular hybridization does not discriminate between methylated and unmethylated DNA, since the methyl group on the cytosine does not participate in base pairing. The lack of a facile way to amplify the methylation information in complex genomic DNA has probably been a most important impediment to DNA methylation research. Therefore, there is a need in the art to improve upon methylation detection techniques, especially in a quantitative manner.
The indirect methods for DNA methylation pattern determinations at specific loci that have been developed rely on techniques that alter the genomic DNA in a methylation-dependent manner before the amplification event. There are two primary methods that have been utilized to achieve this methylation-dependent DNA alteration. The first is digestion by a restriction enzyme that is affected in its activity by 5-methylcytosine in a CpG sequence context. The cleavage, or lack of it, can subsequently be revealed by Southern blotting or by PCR. The other technique that has received recent widespread use is the treatment of genomic DNA with sodium bisulfite. Sodium bisulfite treatment converts all unmethylated cytosines in the DNA to uracil by deamination, but leaves the methylated cytosine residues intact. Subsequent PCR amplification replaces the uracil residues with thymines and the 5-methylcytosine residues with cytosines. The resulting sequence difference has been detected using standard DNA sequence detection techniques, primarily PCR.
Many DNA methylation detection techniques utilize bisulfite treatment. Currently, all bisulfite treatment-based methods are followed by a PCR reaction to analyze specific loci within the genome. There are two principally different ways in which the sequence difference generated by the sodium bisulfite treatment can be revealed. The first is to design PCR primers that uniquely anneal with either methylated or unmethylated converted DNA. This technique is referred to as “methylation specific PCR” or “MSP”. The method used by all other bisulfite-based techniques (such as bisulfite genomic sequencing, COBRA and Ms-SNuPE) is to amplify the bisulfite-converted DNA using primers that anneal at locations that lack CpG dinucleotides in the original genomic sequence. In this way, the PCR primers can amplify the sequence in between the two primers, regardless of the DNA methylation status of that sequence in the original genomic DNA. This results in a pool of different PCR products, all with the same length and differing in their sequence only at the sites of potential DNA methylation at CpGs located in between the two primers. The difference between these methods of processing the bisulfite-converted sequence is that in MSP, the methylation information is derived from the occurrence or lack of occurrence of a PCR product, whereas in the other techniques a mix of products is always generated and the mixture is subsequently analyzed to yield quantitative information on the relative occurrence of the different methylation states.
MSP is a qualitative technique. There are two reasons that it is not quantitative. The first is that methylation information is derived from the comparison of two separate PCR reactions (the methylated and the unmethylated version). There are inherent difficulties in making kinetic comparisons of two different PCR reactions. The other problem with MSP is that often the primers cover more than one CpG dinucleotide. The consequence is that multiple sequence variants can be generated, depending on the DNA methylation pattern in the original genomic DNA. For instance, if the forward primer is a 24-mer oligonucleotide that covers 3 CpGs, then 23=8 different theoretical sequence permutations could arise in the genomic DNA following bisulfite conversion within this 24-nucleotide sequence. If only a fully methylated and a fully unmethylated reaction is run, then you are really only investigating 2 out of the 8 possible methylation states. The situation is further complicated if the intermediate methylation states lead to amplification, but with reduced efficiency. Therefore, the MSP technique is non-quantitative. Therefore, there is a need in the art to improve the MSP technique and change it to be more quantitative and facilitate its process to greater throughput. The present invention addresses this need for a more rapid and quantitative methylation assay.